caused its extermination. If many allied forms be developed
from the successful intruder, many will have to yield their places;
and it will generally be the allied forms, which will suffer from some
inherited inferiority in common. But whether it be species belonging
to the same or to a distinct class, which have yielded their places to
other modified and improved species, a few of the sufferers may often
be preserved for a long time, from being fitted to some peculiar line of
life, or from inhabiting some distant and isolated station, where they
will have escaped severe competition. For instance, some species of
Trigonia, a great genus of shells in the secondary formations, survive
in the Australian seas; and a few members of the great and almost
extinct group of Ganoid fishes still inhabit our fresh waters.
Therefore, the utter extinction of a group is generally, as we have
seen, a slower process than its production.

With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period, and of
Ammonites at the close of the secondary period, we must remember what
has been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been
much slow extermination. Moreover, when, by sudden immigration or by
unusually rapid development, many species of a new group have taken
possession of an area, many of the older species will have been
exterminated in a correspondingly rapid manner; and the forms which thus
yield their places will commonly be allied, for they will partake of the
same inferiority in common.

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it
be at our presumption in imagining for a moment that we understand
the many complex contingencies on which the existence of each species
depends. If we forget for an instant that each species tends to increase
inordinately, and that some check is always in action, yet seldom
perceived by us, the whole economy of nature will be utterly obscured.
Whenever we can precisely say why this species is more abundant
in individuals than that; why this species and not another can be
naturalised in a given country; then, and not until then, we may justly
feel surprise why we cannot account